1. Field of the Invention
This invention relates to internal combustion engine aftertreatment systems and more particularly relates to protection of soot filters.
2. Description of the Related Art
Environmental concerns have motivated the implementation of emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of diesel particulate matter (PM), nitrogen oxides (NOx), and unburned hydrocarbons (UHC). Catalytic converters implemented in an exhaust gas after-treatment system have been used to eliminate many of the pollutants present in exhaust gas. However, to remove diesel particulate matter, typically a diesel particulate filter (DPF) must be installed downstream from a catalytic converter, or in conjunction with a catalytic converter.
A common diesel particulate filter comprises a porous ceramic matrix with parallel passageways through which exhaust gas passes. Particulate matter subsequently accumulates on the surface of the filter, creating a buildup which must eventually be removed to prevent obstruction of the exhaust gas flow. Common forms of particulate matter are ash and soot. Ash, typically a residue of burnt engine oil, is substantially incombustible and builds slowly within the filter. Soot, chiefly composed of carbon, results from incomplete combustion of fuel and generally comprises a large percentage of particulate matter buildup. Various conditions, including, but not limited to, engine operating conditions, mileage, driving style, terrain, etc., affect the rate at which particulate matter accumulates within a diesel particulate filter.
Accumulation of particulate matter typically causes backpressure within the exhaust system. Excessive backpressure on the engine can degrade engine performance. Particulate matter, in general, oxidizes in the presence of NO2 at modest temperatures, or in the presence of oxygen at higher temperatures. If too much particulate matter has accumulated when oxidation begins, the oxidation rate may get high enough to cause an uncontrolled temperature excursion. The resulting heat can destroy the filter and damage surrounding structures. Recovery can be an expensive process.
To prevent potentially hazardous situations, it is desirable to oxidize accumulated particulate matter in a controlled regeneration process before it builds to excessive levels. To oxidize the accumulated particulate matter, temperatures generally must exceed the temperatures typically reached at the filter inlet. Oxidation temperatures will be achieved under normal operating conditions in some applications, although in others, additional methods to initiate regeneration of a diesel particulate filter must be used. In one method, a reactant, such as diesel fuel, is introduced into an exhaust after-treatment system to generate temperature and initiate oxidation of particulate buildup in the filter. Partial or complete regeneration may occur depending on the duration of time the filter is exposed to elevated temperatures and the amount of particulate matter remaining on the filter. Partial regeneration, caused either by controlled regeneration or uncontrolled regeneration, can contribute to irregular distribution of particulate matter across the substrate of a particulate filter.
Controlled regeneration traditionally has been initiated at set intervals, such as distance traveled or time passed. Interval based regeneration, however, has not proven to be totally effective for several reasons. First, regenerating a particulate filter with little or no particulate buildup lessens the fuel economy of the engine and unnecessarily exposes the particulate filter to destructive temperature cycles. Second, if particulate matter accumulates significantly before the next regeneration, backpressure from blockage of the exhaust flow can negatively affect engine performance. In addition, regeneration (intentional or unintentional) of a particulate filter containing large quantities of particulate buildup can become uncontrolled and potentially cause filter failure or the like. Consequently, many particulate filters regenerated on a set interval must be replaced frequently to maintain the integrity of an exhaust gas after-treatment system.
Recently, attempts have been made to estimate the amount of particulate matter accumulated in a particulate filter in order to respond more efficiently to actual particulate buildup, such as, in one widely used method, through differential pressure across a diesel particulate filter. These attempts, however, often do not account for variations in engine operating conditions, sensor noise-to-measurement levels, exhaust flow estimate errors, and unevenly distributed particulate accumulation. In many cases they also integrate errors over time and deviate from real soot loading conditions.
Some of these problems have been overcome by combining model-based soot estimates with sensor-based soot estimates. While this approach has dramatically improved the fuel economy and durability of soot filters in aftertreatment applications, there are still circumstances where the new approaches fail to protect the soot filter.
In some applications, the soot filter heats up and oxidizes soot in the middle of the filter, while the periphery of the soot filter does not achieve a high enough temperature to regenerate completely. While a combined soot load model and sensor approach may yield the correct overall soot loading, the periphery soot concentrations may be building over time and can lead to an uncontrolled regeneration event.
In other applications, the vehicle may generally oxidize soot without any active soot regeneration. For example, the vehicle may generate enough temperature and NO2 that the overall soot loading never requires a high temperature oxygen-based regeneration. However, the steady state soot removal that occurs may not clean the entire filter and such applications are vulnerable to high soot concentrations in local areas within the filter. If the vehicle initiates a high temperature event, which might occur in a long mountain climb for example, the localized concentrations of soot may trigger an uncontrolled regeneration event.
Finally, some applications may spend extended periods of time at low flows. In the current technology, direct soot filter estimators such as differential pressure sensors require significant exhaust flow rates to be reliable even when the soot on the filter is ideally distributed. The mixed sensor and model-based approaches can keep a soot loading estimate reliable for hours after the sensor measurement is no longer reliable, but eventually the confidence in the integrating model must degrade in applications that do not allow any direct soot measurements for long periods.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that provide additional protections for soot filters based on the current state of the art. Beneficially, such an apparatus, system, and method, particularly when applied to an exhaust gas after-treatment system, would enable effective protection and regeneration of the soot filter without degrading the fuel economy of the application or interfering with the base control and regeneration mechanisms of the application.